Calibrating Gas Custody Transfer Flow Computers Application Note

Calibrating gas custody transfer flow computers Application Note

Gas custody transfer flow computers that calculate flow by measuring the differential pressure across a flow restriction, such as an orifice plate or other differential pressure flow device, require special calibration to perform at optimum accuracy. In custody transfer applications where the buying and selling of commodities like natural gas is involved, calibration checks are performed frequently as a matter of fiduciary responsibility. For the purpose of this application note, the use of gas custody transfer flow computers in the natural gas transmission industry is referenced. Flow computers need multiple measurements to calibrate each device. In the normal application, three measurements are made: volumetric flow, static (line) pressure and temperature. A calculation is performed using this data to determine the actual mass of the gas flowing through the pipeline.

The Fluke 721 Precision Pressure Calibrator has

721 PRESSURE special features that support the complete calibration CALIBRATOR of natural gas multi-variable electronic flowmeters F1 F2 F3

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V mA 30V 24mA and other types of flow computers. With two internal 7.21psi MAX COM pressure ranges, external Fluke 750P pressure mod- ules and an optional precision RTD probe, all of the calibrations required for the flow computer can be performed with just one instrument. Pump The Fluke 721 is available with two built-in PRT Probe pressure ranges from from 16 psi/1 bar up to Close Close 5000 psi/345 bar. For this application, the valves valves configuration with the low pressure sensor (P1) 16 psi/1 bar and high pressure sensor (P2) of 1500 psi/100 bar is frequently the best fit. Since the

Fluke 721 has an accuracy specification as a percent Fluid Fluid of full scale, it is important to closely match the full stream stream scale of the calibrator to the scale of the application in order to get the best performance. (See sidebar on system accuracy calculation and importance of maintaining an adequate accuracy ratio). Figure 1. Sample flow transmitter with calibration equipment attached. In addition to the calibrator itself, a high and low pressure calibration pressure source will be needed. An accessory RTD probe is also required for measuring temperature. An appropriate low Gas custody transfer flow computer operational theory pressure source with 0.01 inH2O of resolution is needed for the low pressure test, and a high pres- Custody transfer flow computers are called by a sure source such as a regulated nitrogen bottle or variety of names including electronic flowmeters hydraulic hand pump are required. One pump style (EFMs) and multi-variable flow computers, but they typically does not work well for both tests or exten- all feature some common principles of operation. sive cleaning is required to switch from hydraulic 1. Volumetric flow measurement uses some type of (oil or water) to pneumatic testing. High pressure flow restriction such as an orifice plate or other pumps typically do not have the desired resolution differential pressure flow device to generate a for the low pressure test. pressure drop. The differential pressure created

From the Fluke Digital Library @ www.fluke.com/library across this pressure drop is measured by the Industrial applications use either the analog flow computer as the primary measurement. It output of the flowmeter (4 to 20 mA) or a digital is based on the principle that flow velocity is output like the HART signal to get data from the proportional to the square root of the pressure flowmeter to a control system or data acquisition drop. The volumetric rate is then calculated from system. the velocity by knowing the diameter of the pipe This analog output is generally not used in in which the gas is flowing. gas pipeline applications. Instead, the flowmeter The measured pressure drop (differential) is is a specialized device that operates standalone typically around 200 inches of water column to measure and record the total mass flow (“WC) or 8 to 10 psi. through the pipeline. The total is periodically 2. To convert volumetric flow to mass flow you “downloaded” from the flowmeter to be used in also need to know the density of the mass per an accounting of gas flow and custody transfer. volume of the flowing media. The flow com- This information is also often sent through wire- puter makes this calculation using 2 additional lessly to a central control point in operations. measurements, plus a range of factors/constants The flowmeter may be packaged with other based on the flowing media. The two additional electronic devices to be able to perform this measurements are the static pressure of the gas function or it may be purpose manufactured, in the pipeline and the temperature of the gas in which is the most common type. the pipeline. The static pressure in these applications How to calibrate the flow computer ranges widely from a low of about 300 psi/ 20 bar to a high of about 2000 psi/138 bar. Each flow computer manufacturer has created The temperature of the gas is usually at a proprietary method of calibration, but they all ambient, so it is within the range of normal use the same general technique, which will be environmental conditions. described here. 3. A final consideration about flow computers is In these proprietary calibrations, the manufac- how they are typically installed and used. turer has provided a software application, which runs on a notebook computer (PC). The PC is

System accuracy determination In order to effectively calibrate an instrument, the therefore generally be disregarded. As an alterna- calibrator used must be more accurate than the tive to having a calibrator with the appropriate instrument by some factor. The factor will vary ratio, users may elect to de-rate the performance according to the application, but it should be as of the instrument to a value four times that of the large as is practical. The minimum factor is gener- calibrator. ally considered to be 3 to 4 times. The common For example, using a calibrator with ± 0.05 % term for expressing this factor is Test Uncertainty accuracy would have a TUR of 4:1 testing an Ratio or TUR. If the calibrator is 4 times more accu- instrument with an accuracy of ± 0.2 %. Due to rate than the device being tested it is referred to as the continual advances in instrument technol- having a TUR of 4:1. ogy, calibration technology may, from time to time, The rationale behind this comes from a tech- fail to provide the necessary TUR to calibrate to nique for the statistical analysis of the error in a the instrument manufacturer’s rated specification. system. This technique is called Root Square Sum Alternately you can tighten the test tolerance to or RSS. To determine the error in a system you take 80 % of the desired specification to gain the same the square root of the sum of the errors squared confidence using a technique called guardbanding. for all elements in the system. Note that this is not The fundamental concept of guardbanding is to the maximum possible error in a system, but is the restrict the Pass/Fail limits applied to a calibration largest error which is statistically likely. test based on a defined criterion. The purpose of This formula describes the calculation, where Et guardbanding is to control the risk of accepting an is the total error and E1, etc. are the errors of the out-of-tolerance unit, or rejecting an in-tolerance individual components of the system. unit. Without guardbanding the result of a test will be Pass or Fail. With guardbanding the result of a test will be Pass, Fail, or Indeterminate. A Pass or 2 2 2 2 EEt =+EE++... Fail test result without guardbanding may change 1 2 n to a result of Indeterminate with guardbanding. For more information on guardbanding refer to the By using a TUR of 4:1, the effect of the error application note “Guardbanding with Confidence” in the calibrator is reduced to a small percentage at www.fluke.com/guardbanding. of the error of the instrument under test and can

2 Fluke Corporation Calibrating gas custody transfer flow computers connected to the serial port or USB port of the flow 2. Differential pressure calibration computer. In this way, the software both instructs a. The differential pressure calibration is the user to connect appropriate signals to the flow performed using atmospheric pressure as a computer (either pressure or temperature) and com- reference, so the low pressure connection of municates that information to the flow computer so the flow computer or pressure transmitter is that calibration errors can be corrected. vented and the high pressure connection on Note: This procedure is a generic description of the calibration pro- the flow computer or transmitter is connected cess using the Fluke-721-1615 as an example. The actual procedure to the low pressure port on the calibrator. will vary based on the original equipment manufacturer’s design and b. Connect the notebook computer (PC) to the instructions, test equipment used and on local process and policy. flow computer serial or USB port. Use the PC Detailed Procedure to initiate the calibration process. c. The PC will instruct the user to apply one or 1. Setup more test pressures to the flow computer or a. Turn on the calibrator and make sure that you transmitter. For example, on a device with a see three measurements, usually [P1], [P2], full scale differential measurement of 200” and [RTD]. If you only see two measurements, WC, the test pressures may be 0, 100 and press F1 [P1/P2] until three measurements 200” WC. In each case, it is not necessary to appear. If you see three measurements, but provide the exact pressure called for since the not [RTD], press F2 [mA/V/RTD] until [RTD] user will also be prompted to enter the actual appears. Refer to the user guide for more pressure applied at each test point. information about calibrator setups. d. Set the hand pump pressure/vacuum con- b. If needed, set the upper display (P1) to use trol to the pressure mode and close the vent inches of water column (in H2O 60 °F) as the valve. Squeeze the pump handles until the engineering unit, the middle display (p2) desired pressure is generated. The vernier or to use psi as the engineering unit and the fine pressure control of the pump can be used bottom display (RTD) to use °F as the engi- to adjust the pressure up or down in small neering unit. Refer to the user guide for more amounts. information about setting engineering units. Note: 200” WC is approximately 7.2 psi. Since the pump can Note: Once the calibrator has been configured to use these easily exceed this pressure, it may be best to apply repeated settings, they become the default unless subsequently changed short squeezes of the pump to allow for better control. The rate by the user. of pressure increase will be affected by the volume of the test system with faster increases when the volume is lower. c. If necessary, zero both pressure displays while vented to atmosphere. Refer to the user e. When the differential calibration is complete, guide for information about zeroing the pres- open the pump vent control and discon- sure displays. nect the calibrator from the flow computer or d. Isolate the flow computer from the process. (It transmitter. is normally installed with a 5 valve manifold. 3. Static pressure calibration Closing the valves on the process side of a. For the static pressure calibration, the test the manifold will isolate it from the process). pressure will normally be applied to either Be sure to follow local policy and procedure the same high pressure port or both the when performing this step. high and low pressure ports simultaneously. Refer to the manufacturer’s instructions for details on the exact method of connection

TEST DC PWR – ++ – + to perform this test. Connect the high pres-

721 PRESSURE CALIBRATOR sure sensor input (p2) to the appropriate port on the flow computer or transmitter and to the high pressure test source such as hand F1 F2 F3 pump or nitrogen bottle. NOTE: if the source ZERO

V mA 30V 24mA has 2 ports, one can be connected to the (p2) MAX COM input and the other to the port on the flow computer/transmitter. b. The PC will instruct the user to apply one or more test pressures to the flow computer/ transmitter. For example, on a device with

Figure 2. Sample set-up for Fluke 721 to low pressure side of flow transmitter.

3 Fluke Corporation Calibrating gas custody transfer flow computers a full scale static pressure measurement of c. This calibration is based on the concept that 1500 psi, the test pressures may be 0, 750 the same temperature is measured at both and 1500 psi. In each case, it is not neces- thermowells and, therefore, the measured sary to provide the exact pressure called for values should be identical. The PC will prompt since the user will be prompted to enter the the user to enter the value observed on the actual pressure applied at each test point. calibrator. c. Use the high pressure test source to generate d. Remove the RTD from the test thermowell. the called for pressures and enter observed The calibration is complete. data when prompted. 5. Wrap up d. When the calibration is complete, carefully a. Follow local policy and procedures and manu- vent the system and disconnect the and facturer’s instructions for returning the flow pressure source. computer to service. 4. Temperature calibration b. If the flow computer has transmitters for a. Calibration of the temperature measurement measurement of differential pressure, static on the flow computer is done with a single pressure and temperature the flow computer temperature point at the pipeline operating interprets the transmitters scaled mA output temperature. via its mA analog inputs. In this instance, if b. A test thermowell is provided adjacent to the the calibration is unsuccessful, the individual in service measuring RTD connected to the transmitters may need calibration and adjust- flow computer or temperature transmitter. ment to remove errors. One last source of Insert the calibrator probe into the test ther- errors to consider in this configuration, which, mowell and allow time for the measurement is often overlooked is the mA input A/D of the to reach a stable value. flow computer which typically has an offset Note: Inserting the probe can be done prior to the pressure and gain adjustment. calibrations if local conditions permit. That allows sufficient time to reach stability.

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4 Fluke Corporation Calibrating gas custody transfer flow computers